Conventionally, a technique is known that measures the distance from a base station to a mobile terminal and a position where a known mobile terminal is located. An example of the positioning and ranging technique is a technique that measures the distance between a base station and a mobile terminal based on the field intensity of continuous waves transmitted and received between the base station and the mobile terminal. The technique utilizes a characteristic that the longer the distance a radio wave propagates; the more the intensity of radio wave attenuates. However, accuracy of the positioning and ranging is low, because the field intensity of the continuous wave changes due to multipath interference.
Another example of the positioning and ranging technique is a technique that uses an impulse. For example, a base station senses an impulse transmitted by a mobile terminal and positioning and ranging is performed based on the time when the base station senses the impulse. Multipath interference is less likely to occur in the impulse compared with a continuous wave. Therefore, the technique that uses an impulse achieves the positioning and ranging with higher accuracy compared with the technique that uses a continuous wave.
However, there is a disadvantage in which the accuracy of the positioning and ranging is still low even in the above technique using an impulse. For example, fluctuation of the field intensity of an impulse causes an error in the time when the base station senses the impulse, and thereby results of the positioning and ranging may also fluctuate.
The FIGS. 22 and 23 illustrate examples of impulse waveforms. Here, it is assumed that the base station receives impulses illustrated in FIGS. 22 and 23 at substantially the same time. A reception sensitivity level of the base station is assumed to be “TH1.” In other words, the base station senses an impulse with field intensity that reaches “TH1.”
Under the condition, when the base station receives the impulse illustrated in FIG. 22, the base station senses the impulse, for example, at time t1. This is because the field intensity of the impulse illustrated in FIG. 22 reaches the reception sensitivity level TH1 at the time t1. On the other hand, when the base station receives the impulse illustrated in FIG. 23, the base station senses the impulse, for example, at time t2. This is because the field intensity of the impulse illustrated in FIG. 23 reaches the reception sensitivity level TH1 at the time t2.
In other words, even when the distance between the base station and the mobile terminal is substantially the same, the base station senses the impulse at different times depending on the field intensity. This leads to a disadvantage in which results of the positioning and ranging fluctuate depending on the field intensity of the impulse.
Recently, there is a receiver that outputs a received impulse to a plurality of comparators, and determines a shape of an impulse waveform based on the time sensed by each comparator. However, the receiver has disadvantages in which an impulse with weak field intensity may not be processed because the impulse is diverged and the circuit becomes complicated.